The present invention relates generally to implantable medical leads and more specifically to leads with rigid electrode head assemblies.
Screw-in leads of the type provided with advancable fixation devices such as screws or barbs generally are provided with relatively rigid electrode head members provided with internal lumens housing the advancable fixation devices. Particularly in the context of screw-in leads, it is common to use a coiled conductor, coupled to a rotatable fixation helix which also serves as a pacing electrode. Such leads are illustrated in U.S. Pat. No. 4,106,512 issued to Bisping. In bipolar screw-in leads, a ring electrode may also be mounted to the electrode head, as illustrated in U.S. Pat. Nos. 5,354,327 issued to Smits. In both unipolar and bipolar leads, the relatively rigid electrode head is typically coupled to a substantially more elastic lead body carrying both a the coiled conductor coupled to the fixation helix and the conductor coupled to the ring electrode.
It is sometimes necessary to remove a chronically implanted lead, which may be adhered to the heart by fibrotic tissue growth For purposes of improved extractability, it has long been recognized that it is desirable to reinforce leads by providing an inextensible or only slightly extensible member extending the length of the lead body to prevent separation of the distal end of the lead body during application of traction to the proximal end of the lead body. Several such reinforcement mechanisms are disclosed in U.S. Pat. No. 5,231,996 issued to Bardy et al. A separate reinforcing member is not necessary if the conductor coupled to the distal or tip electrode of the lead is coupled to an inextensible conductor, as in U.S. Pat. No. 5,246,014, issued to Williams et al. This expedient is, unfortunately, not readily available in the case of a lead in which rotation of the fixation helix is accomplished by means of the conductor. In order to provide torque transfer, such leads typically employ coiled conductors, as noted above, which are inherently extensible. The presence of the coil conductor typically will not prevent separation of the lead body from the electrode head as a result of traction applied to the proximal end of the lead.